In the past decades, tremendous progress has been made in InGaN/GaN light emitting diodes (LEDs). One remaining issue is that the performance is still limited by the p-type GaN layer. Due to the low doping concentration and low hole mobility of p-type GaN (poor electric conductivity), the current crowding effect becomes very significant especially at high-current operation. The direct consequences of the current crowding underneath the p-type GaN electrode include creating a high localized heat and localized carrier concentration. This has caused the increase of non-radiative recombination rates in the multiple-quantum wells (MQWs) and thus degrading the optical output power and external quantum efficiency (EQE).
In order to alleviate the undesirable current crowding effect, several technologies have been developed. Sun et. al. proposed the current spreading layer by heavily doping the semiconductor layers (layer 3, 5 and 7 in FIG. 1). The heavily doped layer enables higher conductivity if excellent crystal quality can be preserved. Later on, it is also suggested by Feng et. al. that the metallic pads (label 221 and 225 in FIG. 2) are inserted in the transparent current spreading layers (label 210, 211 and 212 in FIG. 2) to symmetrize the lateral current as indicated in FIG. 2. Particularly, current crowding effect can be suppressed if a highly resistive layer is included, such as undoped Al0.7Ga0.3As (layer 32 in FIG. 3) utilized in the Zn-doped p-type Al0.7Ga0.3As (layer 34 in FIG. 3) for the vertical GaAs based LEDs, which is shown in FIG. 3. Meanwhile, it is reported that current can be homogeneously distributed by considering the physical principle of inter-band tunnelling, and hereby, the transparent current spreading (layer 52 in FIG. 4) on the ultra-thin n+-GaN layer (layer 53 in FIG. 4) for a better current spreading is developed and shown in FIG. 4. On a separate proposed technology, a trench (label 180 in FIG. 5A) is designed in the current path of LEDs to locally block the current and improve the current distribution as shown in FIGS. 5A and 5B. On the other hand, hexagonal III-V nitride grown along polar orientation is featured with strong polarization fields, which are able to form a two-dimensional electron gas (2DEG) and two-dimensional hole gas (2DHG) with high sheet charge density in the heterojunction (i.e., AlGaN/GaN), and this feature can serve as a current spreading layer, as shown in FIG. 6. Last but not least, the current spreading layer can also be achieved by combining undoped AlGaN, undoped GaN, n-type AlGaN and n-type GaN (label 4 in FIG. 7), which not only employs the feature of 2DEG but also generates the energy band variation through alloying technology.